Materials with cavities, such as e.g. honeycomb core panels, can combine excellent mechanical properties with a low mass, making them attractive for application in transport and machine design. However, the high stiffness to mass ratio of these lightweight panels may result in unsatisfactory dynamic behaviour in that it may impair the panels' ability to reduce noise and vibration levels.
A variety of methods exists to insulate against sound transmission. These methods rely often on the acoustic mass law or the addition of an absorptive layer. The first method results in heavy materials, where the second method leads to thick materials, since to be efficient, the thickness of the absorptive layer should be in the same order of magnitude as the acoustic wavelength, making this last method only effective for high-frequency attenuation.
Another approach uses acoustic resonators based on the Helmholtz resonance principle. This phenomenon exists when an air cavity is connected to the atmosphere (air surrounding the structure) through a smaller connection (called the neck). The mass of the air in the neck can resonate on the stiffness of the air cavity, causing dissipation of energy in a specific frequency region. Although allowing very effective sound attenuation in certain frequency regions, these materials are very sensitive for contamination, since the characteristics of the connections between cavity and surrounding environment are crucial and may not be contaminated.
The degradation of performance due to contamination is the main weakness of methods as described in U.S. Pat. No. 7,854,298 B2 or JP2006337886 which are based on both Helmholtz resonances and acoustic impedance of material, which are both very sensitive for contamination.
To reduce structural vibrations most often tuned vibration absorbers are used. The addition of a local structural resonator can, in certain frequencies, trap the most of the movement of the material in a localized mode. The localization of movement is the main principle behind these tuned vibration absorbers. This local resonator is an addition to the structure at a certain point and prevents the movement of the structure at that point at a certain frequency.
This tuned vibration absorbers is a method for local vibration suppression while for acoustic radiation reduction, global vibration reduction is necessary. By using multiple tuned vibration absorbers, one can try to expand the local method to a global method to reduce both vibration and acoustic radiation.
In U.S. Pat. No. 6,576,333 B2 a method is described for creating sound isolation which comprises the inclusion of rigid high density particles in an inclusion of soft material to act as localized resonators. This method however has the disadvantages of changing the mechanical properties of the structure, the addition of considerable weight to the structure and being difficult to manufacture.
In Patent Application Publication No. 2004/0154418 A1 a similar method is used in which instead of an inclusion of coated spheres, addition of a flexible and heavy polymer is used. This solution poses the same problems concerning weight and producability.
In Applied Physics Letters 2010 (96) 041906, Yang et al. propose resonating membranes with high mass inclusions. Here the low acoustic transmission is not found at the resonance frequency of the local resonator but in between two resonance modes of the membrane. This because rather relying on localizing movement as in tuned vibrators, this method relies on the existence of localized modes which have imaginary acoustic wavelengths and are thus not able to radiate acoustically.
In Journal of Sound and Vibration 2011 (330) 2536-2553, Liu et al. propose a method for structural attenuation by the deformation of internal inclusion of high mass spheres with rubber coating in a lattice. The main weakness of this method is that it is only effective for in-plane motion and thus not effective for tackling the common noise and vibration problems. Furthermore the resonance of these structures relies again on a combination of materials which is difficult to produce. The tuning of the attenuated frequencies will have a huge impact on the stiffness of the overall structure and the addition of high density spheres lead to a high increase of mass.
Locally resonant structures can have an impact in the field of focusing and confining light with acoustic meta materials and the technique can be used for blocking in-plane waves, as described by Martinsson et al. in Quarterly of Mechanics and Applied Mathematic 2003 (56) 45-64.